What is a good gift or a 4 year old girl? Money is no object, but I don't want to go crazy or get anything that would make her parents feel uncomfortable. Likewise what is a good gift for a 5 year old boy? They are brother and sister. 
How about a nice outfit, one for each of them? You'll drop about 100 bucks total.
Or you can buy them each a dvd of something they like to watch.